Rapids occur in a fast-moving stream littered with large, hard rock formations.
Water erodes soft land faster than hard, so the rocks remain standing as incomplete barriers. The stream’s fast-moving water rushes around the rocks, often foaming into “white water.”
The flow of water is split into various streams and then reconnects as it rounds the rocks.
Old rivers do not usually have rapids since, over time, their rushing water has eroded any harriers.